As downsizing of information-related devices, communication devices, and the like has been achieved, lithium ion secondary batteries having high energy density and being capable of achieving downsizing and light weights are widely used as power sources for such devices. Also in the fields of electric vehicles (EV) and natural energy power generation, secondary batteries have attracted attention. Accordingly, quick charge for shortening a charge time of a secondary battery is being desired.
As a charge method for secondary batteries, a charge method using constant current or constant electric power has been adopted. For example, in such a charge method, charge is performed using constant current until the voltage of a secondary battery reaches a set voltage, and thereafter, the current value is controlled so as to keep the set voltage.
To perform quick charge by the above charge method, the current value of the constant current may be set high in a period (constant-current charge period) in which charge is performed using the constant current. However, if the current value is high, the performance of storing electricity in a secondary battery, such as a battery capacity and an internal resistance are greatly deteriorated. In addition, deterioration of the storage battery is quickened, and thus, the lifetime of the storage battery is shortened.
To prevent the lifetime of a storage battery from being shortened, a method in which the value of constant current is adjusted on the basis of the characteristics of a secondary battery is known. For example, a method is known in which a constant-current charge period is divided into a plurality of sections on the basis of the internal resistances or the like of the electrodes in a secondary battery and the value of constant current is adjusted in each section. However, the characteristics such as the internal resistances of electrodes vary according to use. Thus, unless the battery characteristics are accurately obtained, quick charge efficiency may be reduced, and further, deterioration of the storage battery may be quickened. In addition, advanced processing for measuring the internal resistances is required so that a process load is increased and the size of the device becomes large. Alternatively, in order to cause an external device to measure the internal resistances, an effort to remove the secondary battery is required.